Fix DragToOverviewTest hang with trace enabled

UIPerformanceTest::SetUpOnMainThread() needs to be called to
start tracing. Otherwise, it hangs on exit when trying to
stop trace.

Bug: 942599
Change-Id: I13045ea5244bc20db8387c11972b2274e4d90f54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566364Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650464}

chrome/browser/ui/ash/drag_to_overview_interactive_uitest.cc

@@ -91,6 +91,7 @@ class DragToOverviewTest : public UIPerformanceTest {
     command_line->AppendSwitch(ash::switches::kAshEnableTabletMode);
   }
   void SetUpOnMainThread() override {
+    UIPerformanceTest::SetUpOnMainThread();
     test::SetAndWaitForTabletMode(true);
 
     if (base::SysInfo::IsRunningOnChromeOS()) {